Television receivers commonly use a type of monolithic integrated-circuit, or IC, which is constructed using bipolar transistors; is designed to follow a surface-acoustical-wave (SAW) or other lumped "block" intermediate-frequency amplifier filter; and comprises a cascade connection of three emitter-coupled differential amplifiers, each exhibiting up to about twentyfold voltage gain, followed by a second detector. Untuned, direct interstage coupling is provided by common-collector-amplifier (or emitter-follower) transistors. Provision is commonly made for automatically controlling the voltage gain of the emitter-coupled differential amplifiers. The second detector may be an envelope detector, but in recent years is more often a synchronous detector or a quasisynchronous detector of the exalted-carrier type. In double-conversion receivers or in an IF amplifier dedicated to generating intercarrier sound, the second detector may be a second mixer for converting a first intermediate frequency to a second intermediate frequency.
U.S. patent application Ser. No. 07/940,220 filed 8 Sep. 1992 by Jack Rudolph Harford and Heung Bae Lee, entitled VARIABLE GAIN AMPLIFIER, describes IF amplifier circuitry modified by the inventor in making embodiments of the invention claimed hereinafter. As is the case in the present application, U.S. patent application Ser. No. 07/940,220 has been assigned to Samsung Electronics Co., Ltd., pursuant to obligations of the inventor(s) to assign his (their) invention(s) at the time the inventions were made. The IF amplifier circuitry described by Harford and Lee meets the automatic gain control range requirements of about 66 dB for a television receiver IF amplifier chain to be met by controlling the voltage gains of first and second emitter-coupled differential amplifiers therein, and a third emitter-coupled differential amplifier therein can be operated with a fixed voltage gain.
Commonly, second detectors are designed to be driven with balanced signals from the IF amplifier chain. Then, the desideratum is for reasonably good matching, at least to within 20 millivolts or so, of the direct bias potentials on which the balanced signals supplied to the second detector are superposed. In previous designs respective low-pass filters, each filter using an off-chip capacitor, extract the direct bias potentials on which these balanced signals supplied to the second detector are superposed. The responses of these low-pass filters are then differentially combined to develop an error signal fed back to the input of the IF amplifier chain, to thereby complete a direct-coupled (d-c) feedback loop for degenerating the error signal. This approach has been found to be disadvantageous. Bringing the fully amplified IF signals off-chip, even to bypass capacitors, increases the risk of undesirable regeneration in the IF amplifier chain. The high voltage gain of the full IF chain under weak-signal conditions and the changes in phase margin that may occur in different portions of the AGC range cause problems of d-c feedback loop stabilization. The reliability of interfaces between the IC and its external environment tends to be lower than the reliability of the electronic circuitry interfaced between. The number of pins required on the IC package affects its cost, and additional pins are often needed for the off-chip capacitors used in the low-pass filters. The off-chip capacitors have to be separately inventoried from the IC during television receiver production.